This invention relates to a nonvolatile semiconductor memory device (also referred to as a nonvolatile memory) and more particularly to a technique which can effectively be applied to a nonvolatile memory that can electrically be written into or those that can electrically be written into and erased from, such as an EPROM (erasable & programmable read-only memory) or an EEPROM (electrically erasable & programmable read-only memory).
An EPROM in which information is electrically written and retained, and in which information is erased by exposing the memory to ultraviolet radiation, is described in the "Hitachi IC Memory Data Book" page 465 to 527, published by Hitachi, Ltd. on March 1987.
In the above-mentioned EPROM, writing information into a memory cell requires application of an adequate voltage for a specified duration to obtain a desired performance that meets the standard requirements. Excessive writing will lead to deterioration of memory cell characteristics.
Writing information into the EPROM is done by using an external EPROM writer. To store specified information in memory cells, the EPROM writer selects a desired memory cell from among a plurality of memory cells forming the EPROM, and applies the specified information to the selected memory cell. The duration in which the specified information is applied to the selected memory cell, i.e., the time during which the information is written (write time) is determined by the EPROM writer. When a write voltage or write time in excess of the standard range is set in the EPROM writer by ignorance or negligence on the part of a user, the memory cell in the nonvolatile memory is applied with an excess charge, resulting in deterioration in the memory cell characteristic. Our examination on the faulty EPROMs returned from the market found that many failed EPROMs were traced to the application of excess charges as the cause of failure.
Memory cells used to form an EEPROM, which can be electrically written and erased, include FLOTOX (floating gate tunnel oxide) type nonvolatile memory cells or MNOS (metal nitride oxide semiconductor) type nonvolatile memory cells. In such nonvolatile memory cells, the address selection switch MOSFET (insulated-gate type field-effect transistor) is connected in series with the memory cell because the threshold voltage is made negative (depletion mode) by an electrical erasure operation.
These FLOTOX and MNOS type memory cells are described in the "Microcomputer Handbook," page 266-267, published by Ohm Co. on Dec. 25, 1985.